I'm fairly certain it would be difficult to use modern 4x5 film holders on your Avus. The Avus uses the most common type of single-sided plate/film holders used also on the Zeiss Maximar and Kodak-Nagel Reccomar, and they are fairly easy to find on eBay or antique photo dealers. (I like the Nagel-made/Kodak-branded holders the best: they will work with cut/sheet film. Most of the others require a film sheath to fit in the slot meant for a glass plate.)
Yours is a 9x12cm, right? If it came with a 135mm or 150mm lens, it probably is. Efke PL 100 sheet film is available in 9x12cm size from several European dealers, and Freestyle in California. The Jobo daylight tank that fits six 4x5" sheets will also work with 9x12cm sheet film.
Can't help you on the lens/shutter questions... which lens is in the Ibsor shutter? If it's a Skopar, it's a Tessar copy, but I frankly don't know if it will fit in other shutters.
Might be easier to fit another lens/shutter. I recently picked up for $9/US in a thrift store a well-worn 9x12cm Zeiss Donata with light-tight bellows & missing a lens, but was happy to find that it will fit both my Kodak Ektar 203mm f7.7 and 127mm f4.7 Ektar lenses with Supermatic shutters. The camera will also close totally with both lenses. (But not with a lenscap on.)
Anyway, sorry I can't answer most of your questions, but 9x12cm plate cameras are a lot of fun to use, and far smaller than any 4x5" camera. I've also got a 9x12cm "BeeBee" camera with a Zeiss 135mm Tessar, and have got some good pictures from it.
--Leigh in Santa Barbara, Calif.